1. Field of the Invention
The present invention relates to a process for producing a dispersion of a pigment having a regulated average particle diameter.
2. Background Art
Ink jet recording is a printing method wherein droplets of an ink composition are ejected and deposited on recording media, such as paper, to perform printing. The feature of the ink jet recording method is that images having high quality with high resolution can be printed at a high speed by means of a relatively inexpensive apparatus.
Ink compositions used in ink jet recording generally comprise water as a main ingredient, a colorant and a wetting agent, such as glycerin, for clogging preventive purposes and the like. A large number of water-soluble dyes have been used as the colorant for the ink composition for ink jet recording, for example, from the viewpoints of high chroma of the coloring material, a wide variety of usable coloring materials, and solubility in water.
On the other hand, because of good lightfastness and waterfastnes of pigments studies have been made in recent years on the use of pigments as, a colorant in ink compositions for ink jet recording, for improving lightfastness and waterfastness of prints. Since pigments are generally insoluble in water, the pigments necessitate the mixing and dispersing process in which the pigment is, for example, mixed together with a resin called a dispersant to stably disperse the pigment in water and then an ink composition is prepared from the dispersant. In order to stably disperse a pigment in an aqueous solvent, it is necessary to study the type and particle diameter of pigments, the type of resins used, dispersing means and the like. Up to now, a large number of dispersion methods and production processes of inks for ink jet recording have been proposed in the art.
In the production of the: ink composition, it is common practice to mix a pigment, a dispersant, and optionally a water-soluble organic solvent together in a suitable dispergator or a mixer to prepare a mixture which is then filtered to remove coarse particles and unnecessary materials. For example, Japanese patent Laid-Open No. 287836/1998 proposes a process for producing an ink jet ink, comprising the steps of: adsorbing a resin onto carbon black; and then removing at least a part of the resin not adsorbed onto the carbon black by ultrafiltration.
So far as the present inventors know, however, any production process of a dispersion of a pigment having a regulated average particle diameter by removing coarse pigment particles through membrane filtration has not been proposed yet in the art.
The present inventors have now found that crossflow membrane filtration can efficiently produce a good dispersion of a pigment having an average particle diameter regulated so as to fall within a given average particle diameter range. Further, the present inventors have found that the ink composition using the pigment dispersion produced by this process is good in excellent ejection stability and can realize images possessing good color reproduction. The present invention has been made based on such finding.
Accordingly, it is an object of the present invention to provide a process for producing a pigment dispersion which can realize an ink composition having excellent ejection stability and capable of yielding good images, especially images possessing excellent color reproduction.
According to a first aspect of the present invention, there is provided a process for producing a dispersion of a pigment having a regulated average particle diameter, said process comprising the steps of:
dispersing the pigment in a solvent to prepare a stock pigment dispersion; and
subjecting the stock pigment dispersion to crossflow membrane filtration to recover a liquid, which is passed through the filtration membrane, or a liquid, which is not passed through the filtration membrane, as a pigment dispersion.
According to a second aspect of the present invention, there is provided a process for producing a dispersion of a pigment having a regulated average particle diameter, said process comprising the steps of:
dispersing the pigment in a solvent to prepare a stock pigment dispersion;
subjecting the stock pigment dispersion to first-stage crossflow membrane filtration to recover a liquid, which is passed through the filtration membrane; and
subjecting said liquid to second-stage crossflow membrane filtration to recover a liquid, which is not passed through the filtration membrane, as a pigment dispersion, the average pore diameter of the membrane used in the first-stage crossflow membrane filtration being larger than the average pore diameter of the membrane used in the second-stage crossflow membrane filtration.
According to a third aspect of the present invention, there is provided a process for producing a dispersion of a pigment having a regulated average particle diameter, said process comprising the steps of:
dispersing the pigment in a solvent to prepare a stock pigment dispersion;
subjecting the stock pigment dispersion to first-stage crossflow membrane filtration: to recover a liquid, which is not passed through the filtration membrane; and
subjecting said liquid to second-stage crossflow membrane filtration to recover a liquid, which is passed through the filtration membrane, as a pigment dispersion,
the average pore diameter of the membrane used in the first-stage crossflow membrane filtration being smaller than the average pore diameter of the membrane used in the second-stage crossflow membrane filtration.